In general, a positron emission tomography (hereafter, referred to as PET) device is used to form an image of a particular human organ or a tumor or used to form an image for diagnosing biochemical phenomena of metabolic activity areas. The PET device generates a tomographic image by marking a radioactive isotope which emits positrons on various basic metabolites and injecting it to a human body, and by detecting, outside the body, gamma ray generated from the interaction between the positron and the metabolites.
A PET image obtained by the PET device generally has low resolution. For this reason, in the past, the low resolution of the PET image has been compensated by using the PET device together with a computer tomography (hereafter, referred to as CT) device. However, a CT image obtained by the CT device generally has a low contrast to soft-tissues.
Therefore, in the past, a positron emission tomography-magnetic resonance imaging (PET-MRI) convergence system has been developed which has a high contrast to soft-tissues of a human body, provides molecular images and functional images and uses the PET device together with a magnetic resonance imaging (MRI) device without radiation exposure.
FIG. 1 is a view showing a cross section of a prior PET-MRI convergence system. Referring to FIG. 1, in the prior PET-MRI convergence system 100, a PET detector 130 and an MRI RF coil 120 are in parallel arranged to have the same field of view (FOV). In other words, the PET detector 130 is located between the MRI RF coil 120 and a magnet bore 101.
The PET-MRI convergence system 100 causes an interaction between PET and MRI. Many noises are hereby generated, which degrades image quality. The noise is caused by MRI, for example, a high magnetic field, high frequency high power interference and low frequency high power interference and the like. The noise is also caused by PET, for example, magnetism distortion, signal-to-noise ratio reduction and Eddy current and the like. Here, particularly, since the high magnetic field caused by MRI and radio frequency energy caused by MRI have much influence on the PET, an RF shielding 140 is placed between the MRI RF coil 120 and the PET detector 130 in order to minimize the influence. However, when the RF shielding 140 is placed, the RF shielding 140 deteriorates the performance of the MRI RF coil 120 and the performance of an RF receiver 150, generates Eddy current in a gradient magnet 110, so that the resolution of an MRI image is degraded.
The RF receiver 150 is fixed and installed on a carrier 160 carrying an examinee 170. In obtaining the MRI image, the installation location of the RF receiver 150 is not considered. However, in obtaining a PET image, the RF receiver 150 may have much influence on the PET image.
Since the PET detector 130 is disposed farther from the circular centric point of the magnet bore 101 than the MRI RF coil 120, a gamma ray emitted from the examinee 170 is attenuated and scattered by the MRI RF coil 120, so that the intensity of the gamma ray is reduced.
Additionally, as shown in FIG. 1, the prior PET-MRI convergence system 100 in which the PET detector 130 and the MRI RF coil 120 are disposed in parallel with each other has a problem in that the size of the outermost magnet bore 101 should be increased in order to obtain the field of views (FOV) of the MRI and PET.
In order to reduce the influence of the PET device on the MRI image, a non-magnetic material may be used in the PET detector 130, an optical amplifier and an electronic circuit and the like of the PET-MRI convergence system 100. However, it is difficult to use the non-magnetic material in all the elements. For example, the non-magnetic material cannot be used in the capacitor and the resistance element of a pre-amplifier disposed inside the magnet bore 101. Therefore, the magnetic component present inside the PET detector 130 affects the main magnetic field uniformity of MRI. Further, the PET detector 130, the RF shielding 140 and other electronic circuits may generate the Eddy current by interacting with the signals generated from the gradient magnet 110. The Eddy current reduces the signal-to-noise ratio of the MRI image and the quality of an image obtained through the PET-MRI convergence system 100 is deteriorated.